1. Field of the Invention
The present invention relates to a silicon single crystal wafer having few crystal defects, as well as to a method for producing it.
2. Description of the Related Art
Along with a decrease in size of semiconductor devices for achieving an increased degree of integration of semiconductor circuits, more severe quality requirements have recently been imposed on silicon single crystals which are grown by the Czochralski method (hereinafter referred to as the CZ method) for use as materials for substrates of semiconductor devices. Particularly, there has been required a reduction in density and size of grown-in defects such as FPDs, LSTDs, and COPs, which are generated during the growth of a single crystal and degrade oxide dielectric breakdown voltage and characteristics of devices.
In connections with the above-mentioned defects incorporated into a silicon single crystal, first are described factors which determine the concentration of a point defect called a vacancy (hereinafter may be referred to as V) and the concentration of a point defect called an interstitial-silicon (hereinafter may be referred to as I).
In a silicon single crystal, a V region refers to a region which contains a relatively large number of vacancies, i.e., depressions, pits, voids or the like caused by missing silicon atoms; and an I region refers to a region which contains a relatively large number of dislocations caused by excess silicon atoms or a relatively large number of clusters of excess silicon atoms. Further, between the V region and the I region there exists a neutral (hereinafter may be referred to as N) region which contains no or few excess or missing silicon atoms. Recent studies have revealed that the above-mentioned grown-in defects such as FPDs, LSTDs, and COPs are generated only when vacancies and/or interstitials are present in a supersaturated state and that even when some atoms deviate from their ideal positions, they do not appear as a defect so long as vacancies and/or interstitials do not exceed the saturation level.
It has been confirmed that the concentration of vacancies and/or interstitials depends on the relation between the pulling rate (growth rate) of crystal in the CZ method and the temperature gradient G in the vicinity of a solid-liquid interface of a growing crystal, and that another type of defect called oxidation-induced stacking fault (hereinafter may be referred to as OSF) is present in ring-shape distribution in the vicinity of a boundary between a V region and an I region, when the cross section vertical to the axis of crystal growth is observed.
When a crystal is pulled through use of a CZ pulling apparatus with a furnace structure (hereinafter occasionaly referred to as hot zone: HZ) having a large temperature gradient G in the vicinity of a solid-liquid interface of the crystal with varying a growing rate from high speed to a low speed along the crystal axis, a defect distribution chart for defects due to crystal growth as shown in FIG. 5 can be obtained.
The defects in the radial cross section can be classified as follows. When the growth rate is relatively high; e.g., about 0.6 mm/min or higher, grown-in defects such as FPDs, LSTDs, and COPs which are believed to be generated due to voids at which vacancy-type points defects aggregate are present at a high density over the entire radial cross section of a crystal. The region where these defects are present is called a xe2x80x9cV-rich regionxe2x80x9d (See FIG. 5(A), line (A) and FIG. 6(A)). When the growth rate is not greater than 0.6 mm/min, as the growth rate decreases the above-described OSF ring is generated from a circumferential portion of the crystal. In such a case, L/D (large dislocation, simplified expression of interstitial dislocation loop) defects such as LSEPDs and LFPDs which are believed to be generated due to dislocation loop are present at a low density outside the OSF ring. The region where these defects are present is called an xe2x80x9cI-rich regionxe2x80x9d (hereinafter occasionally referred to as L/D region). Further, when the growth rate is decreased to about 0.4 mm/min or less, the above-described OSF ring shrinks to the center of a wafer and disappears, so that the I-rich region spreads over the entire cross section of the wafer (See FIG. 5, line (C), FIG. 6(C)).
Further, there has been recently found the existence of a region, called a N (neutral) region, which is located between the V-rich region and the I-rich region and outside the OSF ring and in which there exists neither defects of FPDS, LSTDs and COPs stemming from voids nor defects of LSEPDs and LFPDs stemming from a dislocation loop. The region has been reported to be located outside the OSF ring, and substantially no oxygen precipitation occurs there when a single crystal is subjected to a heat treatment for oxygen precipitation and the contrast due to oxide precipitates is observed through use of an X-ray beam. Further, the N-region is on an I-rich region side, and is not rich enough to cause formation of LSEPDs and LFPDs (See FIG. 5, line (B), FIG. 6(B)).
It is proposed that the N-region that is present only partly in the wafer when using a conventional CZ pulling apparatus can be expanded by improving temperature distribution in the furnace of the pulling apparatus, controlling a pulling rate so that V/G value may be 0.20-0.22 mm2/xc2x0 C. min, in which V is a pulling rate (mm/min), and G is an average intra-crystal temperature gradient (xc2x0C./mm) along the pulling direction from a melting point of silicon to 1300xc2x0 C. (xc2x0C./mm) in the entire surface of the wafer and in full length of the crystal (Japanese Patent Application Laid-open (kokai) No. 8-330316).
However, for producing such a single crystal that the region having a very low defect density is expanded to the entire crystal, the producing condition should be controlled in a very narrow range, since the region is limited to the N-region on the side of I-rich region. Particularly, not in a test machine but in a machine for actual production, it is difficult to control precisely, and therefore, there is a problem in productivity, and such a method is not practical.
In a current method of general silicon single crystals, when the growing rate is intentionally changed along the crystal axis from high speed to low speed as shown in FIG. 5, the following types of crystals can be obtained as shown in FIG. 6: (A) crystals having V-rich region in the entire cross section, (B) crystals having both V-rich region and N-region, (C) crystals having I-rich region in the entire cross section (occasionally referred to as L/D rich region type crystal) and (D) crystals having both V-rich region and I-rich region (not shown). Therefore, the growing rate is controlled along the crystal axis to produce a crystal having a quality suitable for intended uses.
The crystals of the type (A) are mass-produced as standard products. The crystal of the type (B), namely V-N coexistent type crystals are produced as improved products of the crystals of the type (A). However, in a device process, yield is low in V-rich region although high in N-region. Accordingly, the crystal of the type (B) is not completely improved. The crystals of the type (C), wherein the entire cross section is occupied by I-rich region are produced as a particle monitor. However, it is not used for fabrication of device, since L/D is detrimental.
The wafers of the types (A), (C) and (D) have a problem that the device yield is lowered by influence of large vacancies, interstitial dislocations or the like remaining on the surface of the wafers when they are used in a device process.
Recently, crystals of type (E) wherein the entire cross section is occupied by N-region (not shown) are proposed. However, they are not practical, since productivity thereof is low. There are also proposed crystals of type (F) wherein the entire cross section is occupied by N-region and OSF ring is generated when being subjected to thermal oxidation or nuclei of OSF ring are present, and neither FPD nor L/D is present in the entire cross section (Japanese Patent Application No. 9-325428). However, there may be present vacancy defects which are finer than FPD. Such defects are detected with Cu decoration. They may cause degradation of oxide dielectric breakdown voltage, and require further improvement.
The present invention has been accomplished to solve the above-mentioned previous problems, and an object of the invention is to produce under a stable manufacture condition a silicon single crystal wafer by CZ method wherein OSF in the ring shape distribution generated when being subjected to thermal oxidation or latent nuclei of OSF is present in a low density, and neither FPD, COP, L/D, LSTD nor defect detected by Cu decoration is present.
To achieve the above-mentioned object, the present invention provides a silicon single crystal wafer produced by Czochralski method wherein OSF in the ring shape distribution generated when being subjected to thermal oxidation or nuclei of OSF are present, but neither FPD, COP, L/D, LSTD nor defect detected by Cu decoration is present in the entire surface of the wafer.
As described above, in the wafer of the present invention, although OSF in the ring shape distribution or latent nuclei of OSF are present, grown-in defects such as FPD or the like are not present. Particularly, defects detected by Cu decoration are not present in the wafer of the present invention.
Preferably, oxygen concentration in the wafer is less than 24 ppma (ASTM""79 value).
There can be thereby obtained a silicon single crystal wafer wherein OSF is not generated although latent nuclei of OSF are present when being subjected to thermal oxidation, neither FPD, COP, L/D, LSTD nor defect detected by Cu decoration is present in the entire cross section.
The present invention also provides a silicon single crystal wafer containing very few defects having a density of OSF of 100 number/cm2 or less generated when the wafer is subjected to the thermal oxidation treatment. The density of OSF is measured by subjecting a silicon single crystal wafer to a heat treatment at 1200xc2x0 C. for 100 minutes and then to a etching treatment with Wright solution.
The above mentioned silicon single crystal wafer can be produced by, for example, a method for producing a silicon single crystal in accordance with the CZ method wherein a silicon single crystal is pulled so that xcex94G may be 0 or a negative value, where xcex94G is a difference between the temperature gradient Gc (xc2x0C./mm) at the center of a crystal and the temperature gradient Ge (xc2x0C./mm) at the circumferential portion of the crystal, namely xcex94G=(Gexe2x88x92Gc), wherein G is a temperature gradient in the vicinity of a solid-liquid interface of a crystal from the melting point of silicon to 1400xc2x0 C.
The present invention provides a method for producing a silicon single crystal in accordance with the CZ method wherein a crystal is pulled with controlling a temperature in a furnace so that xcex94G may be 0 or a negative value, where xcex94G is a difference between the temperature gradient Gc (xc2x0C./mm) at the center of a crystal and the temperature gradient Ge (xc2x0C./mm) at the circumferential portion of the crystal, namely xcex94G=(Gexe2x88x92Gc), wherein G is a temperature gradient in the vicinity of a solid-liquid interface of a crystal from the melting point of silicon to 1400xc2x0 C., and with controlling a pulling rate in a range between a pulling rate corresponding to a minimum value of the inner line of OSF region and a pulling rate corresponding to a minimum value of the outer line, when OSF region is generated in an inverted M belt shape in a defect distribution chart which shows a defect distribution in which the horizontal axis represents a diameter of the crystal and the vertical axis represent a pulling rate.
The OSF region means a distribution of OSF ring in a direction of a crystal growing axis.
As described above, when the crystal is pulled with controlling the temperature in the furnace so that the difference xcex94G between the temperature gradient at the center of a crystal and the temperature gradient at the circumferential portion of the crystal wherein G is a temperature gradient in the vicinity of a solid-liquid interface of a crystal from the melting point of silicon to 1400xc2x0 C., may be 0 or negative value, and with controlling a pulling rate to be in the range defined above, using the defect distribution chart of FIG. 1 which is prepared by analyzing results of experiments and investigation, silicon single crystal wherein OSF in the ring shape distribution or latent nuclei of OSF are present when being subjected to thermal oxidation and neither FPD, COP, L/D, LSTD nor defect detected by Cu decoration is present in the entire surface of the wafer can be produced.
In one embodiment of the present invention, the accuracy of the pulling rate during growth of the crystal is in the range of an average of values which are calculated at every tenth centimeters of growth length of the constant diameter portion of the crystal (the straight body part of the single crystal)xc2x10.01 [mm/min]. Such an accurate pulling rate makes it possible to stably produce a silicon single crystal under the above-mentioned condition.
In order to control the temperature in the furnace, an annular solid-liquid interface insulator is provided in the pulling apparatus such that a gap of 5-10 cm is formed between the lower end of the insulator and the surface of the silicon melt.
The temperature in the furnace is thus controlled so that the difference xcex94G between the temperature gradient Gc [xc2x0C./cm] at the center of a crystal and the temperature gradient Ge [xc2x0C./cm] at the circumferential portion of the crystal [xcex94G=(Gexe2x88x92Gc)] may be 0 or a negative value, namely the temperature gradient at the circumferential portion of the crystal may be the same as, or lower than the temperature gradient at the center of the crystal. The defect distribution is in an inverted M shape in the above case.
In the silicon single crystal wafer produced by slicing the silicon single crystal produced by the above-mentioned methods, OSF genereated in the ring shape distribution or nuclei of OSF are present when being subjected to thermal oxidation, but neither FPD, COP, L/D, LSTD nor defects detected by Cu decoration is present on the entire surface of the wafer.
As described above, according to the present invention, there can be easily produced a silicon single crystal wafer wherein OSF in the ring shape distribution generated when being subjected to thermal oxidation or latent nuclei of OSF are present, and neither FPD, COP, L/D, LSTD nor defect detected by Cu decoration is present in the entire surface of the wafer, namely N-region is expanded to the maximum. Furthermore, the silicon single crystal wafer having no OSF and substantially no defects in the entire surface of the wafer can be produced by lowering oxygen concentration.